Generally, solder bumping includes placing solderable material on bonding pads of a chip substrate and then reflowing the solderable material to form attached solder bumps on the bonding pads. A number of solder bump forming methods exist according to the prior art. In a common method, illustrated in FIGS. 1 and 2, a stencil mask printing method 100, 200 is used wherein solder paste 102, is squeezed through openings in a metal mask 104 and openings in a solder resist layer 106 onto bonding pads 110 located on the surface of a substrate 108. After mask 104 removal, the placed solder 102 is heated or reflowed to melt the solder 102 to form bumps 202 on the bonding pads 208 of the substrate 206. The stencil printing method and associated methods may not be suited for future platforms that require decreasing solder pitch and bump sizes. Stencil mask printing at fine pitches (i.e.—less than about 150 μm) results in mask lift-off, missing bumps, bump voiding, and excessive bump height variation, thus negatively affecting die attachment yields.
Alternative bumping techniques are being investigated to overcome these limitations. Microball delivery is one such technique that has elicited interest owing to its ability to produce relatively void-free bumps with excellent bump height control. However, delivery methods available today may be restricted to substrates with a single solder resist opening (SRO) size. Current microball techniques cannot deliver microballs of different sizes to mixed pitch and mixed size SROs without using a multiple delivery and placement process, with concomitant increased costs and lowered throughput time. For example, substrates with mixed SROs may currently require a delivery process for each SRO/microball size. Separate delivery of different-sized microballs may also require multiple reflow steps. Solutions for improved microball delivery and placement in a more cost and time efficient manner are needed.